It is known from the state of the art to provide base stations with smart antenna arrays which enable the output of fully steerable downlink beams. When employed for a user specific digital beamforming, a beamformer of such a smart antenna array is e.g. able to weight phase angle and/or amplitude of the transmitted signals in a way that the direction of the beam is adapted to move along with a terminal through the whole sector of coverage of the antenna array.
In order to be able to move a downlink beam according to the movement of a terminal, the base station has to determine the direction in which the terminal can be found. This can be achieved by estimating the azimuth direction of arrival of the uplink signals received by the base station from the respective terminal. For receiving uplink signals, base stations often employ a fixed beam reception system, the fixed beams being evaluated for estimating the direction of arrival of the uplink signals.
For illustration, FIG. 1 shows an example of an architecture in a base station used for the processing of signals from a single user for estimating the direction of arrival (DoA).
The part of the base station depicted in FIG. 1 comprises an uplink digital beam matrix 11 connected at its inputs to a uniform linear antenna array (ULA) with eight receiver antennas (not shown). The output of the uplink digital beam matrix 11 is connected via means for standard RAKE processing 12 to means for estimating the direction of arrival of uplink signals 13. The means for estimating the direction of arrival 13 are connected on the one hand to further components of the base station that are not shown. On the other hand, they are, connected to processing means 14 suited for spreading and weighting of signals. The processing means 14 receive as further inputs signals from means for downlink bit processing 15 and output signals to means for user-specific digital beamforming 16. The outputs of the means for user-specific digital beamforming 16 are connected to eight transmit antennas (not shown). The means for standard RAKE 12, for estimation of the DoA 13, for downlink bit processing 15 and the processing means 14 are used for digital base-band processing.
Signals entering the base station via the receive antennas are first processed in the digital beam matrix 11. The digital beam matrix 11 is an M×M matrix, where M is the number of antenna elements, i.e. M=8 in the described example. The digital beam matrix 11 generates from the received signals fixed reception beams in eight different directions. With the digital beam matrix 11 and the uniform linear antenna array (ULA), orthogonal beams (butler matrix) or an arbitrary set of non-orthogonal beams can be generated. The generated beams are input to the means for standard RAKE 12.
After a processing on the chip level by the means for standard RAKE 12, the beams are evaluated in the means for estimation of the direction of arrival 13 in order to be able to determine the best direction for transmission of downlink signals. The direction of arrival of the uplink signals can be estimated by simply measuring the power from each beam. In particular, the power in the pilot symbols in the channel estimate can be determined. The beam direction of the beam with the highest uplink power, averaged over fast fading, is considered as the direction of arrival, to which the downlink beam is to be directed. Alternatively, the direction of arrival can be estimated with any other known method for determining the direction of arrival in the beam space. The means for estimation of the direction of arrival 13 provide the processing means 14 with power control and weight information for forming the downlink beams corresponding to the determined direction of arrival.
In addition, further elements in the means for estimation of the direction of arrival 13 forward soft bits, including the data signals transmitted by the terminal, to the components not depicted in the figure.
Hard bits constituting signals that are to be transmitted from the network to the terminal are processed, e.g. encoded, by the means for downlink bit processing 15 and forwarded to the processing means 14. The processing means 14 are able to spread and weight those signals according to the information received from the means for estimation the direction of arrival 13. The thus processed signals are transmitted to the means for user-specific digital beamforming 16 which transmit the signals via the transmit antennas in a downlink beam directed to the determined direction of arrival of the uplink signals.
With this method, the estimation of the uplink direction of arrival is based on a rough resolution grid in the form of the fixed beams. That means, even though in the downlink the transmission beam can be steered continuously with arbitrary resolution, the accuracy of the downlink beamforming is limited to the uplink beam spacing. This accuracy is not adequate for downlink beam steering, if the number of beams is equal to the number of columns in the smart antenna array. Even if the direction of arrival resolution is improved as the number of reception beams is increased by increasing the number of receive antennas, the angular resolution is not adequate with 4-8 beams/antennas. In the uplink, the angular resolution is approximately 30° with 4 beams and approximately 15° with 8 beams.
FIGS. 2a-d show this angular distribution of the fixed uplink beams for different constellations. FIG. 2a is a diagram with the amplitude beam pattern over the azimuth angle in degrees of four orthogonal beams resulting from a 4-antenna array. FIG. 2b is a diagram with the corresponding amplitude beam pattern of eight orthogonal beams of a 8-antenna array. In contrast, FIG. 2c is a diagram with the amplitude beam pattern of four non-orthogonal beams of a 4-antenna array and FIG. 2d a diagram with the amplitude beam pattern of eight non-orthogonal beams of a 8-antenna array.
Alternatively to basing the estimation of the direction of arrival on the power of the fixed beams, the direction of the downlink beam can be selected by transforming the channel estimates back to the element domain. To this end, the beamformed signals are multiplied by an inverted digital beam matrix to obtain the element space signals. Then, any known direction of arrival techniques is used in the element space. However, for practical implementations this method leads to an excessive amount of computations.